Spinodal Decomposition During Anion Exchange in Colloidal Mn2+-Doped CsPbX3 (X = Cl, Br) Perovskite Nanocrystals

Manganese(II)-doped cesium–lead–chloride (Mn2+:CsPbCl3) perovskite nanocrystals have recently been developed as promising luminescent materials and attractive candidates for white-light generation. One approach to tuning the luminescence of these materials has involved anion exchange to incorporate Br–, but the effects of anion exchange on Mn2+ speciation in doped metal-halide perovskites is not well understood at a microscopic level. Here, we use a combination of X-band electron paramagnetic resonance (EPR) and photoluminescence spectroscopies to monitor the Mn2+ dopants in Mn2+:CsPbCl3 nanocrystals during Cl– → Br– anion exchange. Analytical measurements show that the nanocrystals retain their Mn2+ over the course of Cl– → Br– anion exchange and they continue to show strong Mn2+ d–d luminescence but, surprisingly, the Mn2+ EPR intensities all but vanish. Further results suggest that Mn2+ ions migrate during anion exchange to form clusters that are still luminescent but show no EPR signal due to antiferromagnetic superexchange coupling. Monte Carlo simulation and analysis of the Mn2+:CsPb(Cl1–xBrx)3 lattice at various halide compositions (x) bolsters this interpretation by indicating a propensity for Mn2+–Cl– units to cluster as the Br– content increases, increasing the probability of the nearest-neighbor Mn2+–Mn2+ interactions. The driving force for this clustering is retention of the stronger Mn–Cl bonds compared to Mn–Br bonds. In addition, modeling predicts spinodal decomposition to form Mn2+-enriched domains even at the end point compositions of x = 0 and 1, with Mn2+ ordering in next-nearest-neighbor positions driven by Coulomb interactions and lattice-strain minimization. These results have important implications for both fundamental studies and applications of doped and alloyed metal-halide perovskites.